This invention relates to arc discharge devices such as segmented plasma excitation and recombination (SPER) devices and, in particular, to low voltage arc formation between adjacent electrodes of such devices.
In Applied Physics Letters, Vol. 36, No. 8, pages 615-617 (1980), W. T. Silfvast, L. H. Szeto and O. R. Wood II describe a new electric discharge device developed for producing laser action in the atomic spectra of a number of metal vapors by a segmented plasma excitation and recombination (SPER) mechanism. This laser includes a number of narrow metal strips (of the lasing species) positioned end-to-end on an insulating substrate in such a way as to leave a small gap between each pair of adjacent strips. The strips are surrounded by either a buffer gas (preferably) or a vacuum and typically are 1 mm thick, 2 mm wide, and 10 mm long (hereinafter "bulk electrodes"). When a high-voltage, high-current pulse is applied to the end strips of this arrangement, a high-density metal-vapor ion plasma is formed in each gap. Once formed, these plasmas (consisting primarily of vaporized strip material) expand essentially hemispherically into a laser cavity, cool in the presence of the background gas (e.g., helium) at low pressure and recombine. Using this laser configuration, pulsed laser action was observed in the near infrared at more than 70 wavelengths between 0.29 and 3.95 .mu.m in 11 elements (Ag, Bi, C, Ca, Cd, Cu, In, Mg, Pb, Sn, Zn), three of which (Mg, Zn, In) had not been observed to exhibit laser action in their neutral spectrum before. Some of these results are reported in the aforementioned APL article; others are reported by W. T. Silfvast, et al in Applied Physics Letters, Vol. 39, No. 3, page 212 (1981) and in Optics Letters, Vol. 7, No. 1, page 34 (1982).
The SPER laser is simple to construct, can be easily scaled in length and volume, has been shown to be capable of long life, and has the potential for high efficiency. It is the subject matter of U.S. Pat. No. 4,336,506 issued on June 22, 1981 and copending application Ser. No. 367,092, filed on Apr. 9, 1982 (now U.S. Pat. No. 4,395,770 issued on July 26, 1983). Both the patent and the application are assigned to the assignee hereof.
Lasing action in a SPER laser is not observed with equal facility with all metals, even at high pressure of the background gas. A figure of merit, M (0&lt;M.ltoreq.1), can be derived which defines the relative ease of achieving lasing action in a metal vapor. M is defined as follows: EQU M=1/kc.rho.T.sup.2 ( 1)
where k is the thermal conductivity of the metal, c is the specific heat of the metal, .rho. is the density of the metal, and T is the temperature of the surface of the metal electrode at which the vapor pressure of the metal is conducive to arc formation (.about.0.1 Torr). Experimentally, metals with M.about.1, such as Cd and Na, have been found to easily produce the segmented metal vapor plasmas that are necessary for lasing action in SPER lasers at low background gas pressures (e.g., 1-10 Torr), whereas metals with M&lt;&lt;1, such as Li, Al, Ca, and Cu, do not even produce segmented plasmas at such low pressures. With these metals as the background pressure is reduced, the discharge current is carried by a discharge in the background gas between non-adjacent electrodes, effectively bypassing the intervening metal vapor arcs, reducing the number of metal vapor plasmas and, hence, lowering the net gain.
In another copending application Ser. No. 367,216 also filed on Apr. 9, 1982 (now U.S. Pat. No. 4,441,189 issued Apr. 3, 1984) and assigned to the assignee hereof, we describe how segmented metal vapor plasma discharges and pulsed lasing action in SPER devices can be achieved, even at relatively low background gas pressures, with metal electrodes of materials having M&lt;&lt;1 provided that the metal strips constitute foil electrodes. These electrodes are sufficiently thinner (typically about 10 times thinner) than bulk electrodes so that discharges occur only between adjacent electrodes, thereby eliminating the short circuiting problem associated with bulk electrodes. Using this foil electrode SPER configuration, we have achieved pulsed laser action in four metals (Li, Al, Ca, and Cu) in which laser oscillation was not possible using bulk electrodes and lower pressures. As a result, we observed recombination laser action on 30 transitions with oscillating wavelengths ranging from 569.6 nm to 5460 nm. Twenty-eight of these transitions had not previously been made to undergo laser oscillation by any excitation means. In addition, we observed segmented vapor plasma discharges in a SPER device with Ni foil electrodes.
In the above-described work the SPER lasers were operated in a pulsed mode; that is, the excitation means applied a relatively short duration (e.g., 5 .mu.sec) electrical pulse. Significantly longer duration electrical signals suitable for continuous wave operation would have generated excessive heat in the electrodes, ultimately causing them to melt. Had the electrodes been so damaged, of course, laser operation would no longer have been possible. However, another of our copending applications, Ser. No. 389,779 filed on June 18, 1982, describes continuous wave operation of a SPER laser in which the pump signal is suitable for continuous wave operation and means are provided for flowing a background gas across the electrodes. Using this laser configuration with Cd strips, continuous wave laser action has been observed for the first time in a metal vapor arc discharge plasma. Laser action occurred in the Cd vapor at 1.40, 1.43, 1.44, and 1.64 .mu.m, as the Cd.sup.+ ions recombined in the presence of a flowing He background gas. Typical input powers of 3-4 A at 20 V produced a measured power output of 0.5 mW, although the laser was believed to be operating near threshold and significantly higher powers should be possible. This technique is applicable to the wide range of visible and infrared recombination laser transitions already achieved in pulsed metal vapor arc plasmas.
As the acronym SPER indicates, this family of devices relies primarily on a recombination mechanism to generate light, whether via spontaneous or stimulated emission. That is, electrons in the plasma collide with ground state atoms of the vaporized electrode material, thereby generating ions. Upon cooling of the plasma, those ions recombine with electrons, thereby generating excited ions (in lower ion stages) and atoms. Transitions between energy lines in these atoms and ions result in light emission.
This recombination mechanism is dominant at relatively low pressures of the background gas (e.g., 1-10 Torr), but at higher pressures (e.g., 40 Torr) hot electrons tend to excite atoms directly into the energy levels from which they radiate. The latter mechanism, known as electron-impact excitation, takes place concurrently with recombination, but the two may be either temporally or spatially separate. For example, recombination radiation tends to be delayed relative to radiation due to electron-impact, and recombination tends to occur farther from the electrode gap (in the expanding plasma) whereas electron-impact light tends to be emitted from a region fairly close to the gap. As discussed hereinafter, a segmented plasma electron-impact device, modified in accordance with our invention, may find application as a UV source to erase EPROMs (Erasable Programmable Read Only Memory).
In contrast with the foregoing light source applications of segmented plasma arc discharge devices, we have also discovered that these devices can be utilized to form layers on a workpiece or to etch layers from a workpiece. These material working techniques, which are described in our copending application Ser. No. 389,780, filed on June 18, 1982 (now U.S. Pat. No. 4,411,733 issued Oct. 25, 1983), entail providing an aperture in the gap between adjacent electrodes and flowing background gas through the gap. The flowing gas causes the ion plasma to take the shape of a beam which strikes the workpiece, thereby depositing a layer thereon or etching a layer therefrom depending on the specific operating conditions.
The excitation means utilized in both the light source and material working segmented plasma arc discharge apparatus typically includes a high voltage supply and a low voltage supply connected in parallel. The high and low voltages are applied sequentially to the device. Illustratively, the high voltage (a few kilovolts) serves to breakdown the gaps between adjacent electrodes. Thereafter, the low voltage (20 V D.C. at a few amperes) serves to sustain the arc discharge (i.e., the plasma).